The present invention relates to a system and method of determining completion of a piston stroke in an automatic transmission wherein a selected gear position is determined by engaging at least two of the friction elements at different timings, which determines whether or not a piston for actuating the first friction element to be engaged last has completed its stroke up to the position where it can start actual engagement.
In the automatic transmission, a power transfer route of a gear transmission mechanism or a gear position is determined by selectively hydraulically actuating or engaging friction elements such as clutch and brake. Engine rotation or torque input to the transmission through a torque converter is converted at a ratio responsive to the selected gear position to produce an output. In short, the automatic transmission changes one gear position to another by switching the friction elements to be engaged, achieving out the shift.
When controlling the degree of development of the shift in accordance with the state of the friction elements which ensure the above shift control, the state of the friction elements should be estimated in the way as disclosed in JP-A 4-366063.
Particularly, in the automatic transmission wherein a selected gear position is determined by engaging two friction elements at different timings, the shift is developed in accordance with the first friction element to be engaged last. Thus, in order to achieve a good shift shock, it s necessary to determine whether or not a piston for actuating the first friction element has completed its stroke up to the position where it can start actual engagement.
However, in the automatic transmission wherein a selected gear position is determined by engaging two friction elements at different timings, a turbine revolution corresponding to a transmission input revolution produces fine variations at engagement of the second friction element occurring immediately before engagement of the first friction element. This can lead to erroneous detection that the piston for actuating the first friction element has completed its stroke up to the position where it can start actual engagement. The occurrence of this problem in the transmission is remarkable at a low oil temperature where a drag produced at engagement of the friction elements is apt to be greater.
It is, therefore, an object of the present invention to provide a system and method of determining completion of a piston stroke in an automatic transmission wherein a selected gear position is determined by engaging at least two of the friction elements at different timings, which can appropriately determine completion of the stroke of a piston for actuating the first friction element to be engaged last has completed its stroke up to the position where it can start actual engagement.
The present invention generally provides an automatic transmission, comprising:
a plurality of friction elements with respective pistons, at least first and second friction elements being engaged at different timings to determine a selected gear position, the second friction element being engaged immediately before the first friction element to be engaged last; and
a controller that controls the friction elements,
wherein the controller determines whether or not the second piston for actuating the second friction element has completed its stroke up to a second predetermined position where the second piston can start actual engagement, and
wherein before determination that the second piston has completed its stroke up to the second predetermined position, the controller fails to determine whether or not the first piston for actuating the first friction element has completed its stroke up to a first predetermined position where the first piston can start actual engagement.
One aspect of the present invention is to provide a method of controlling an automatic transmission with a plurality of friction elements with respective pistons, at least first and second friction elements being engaged at different timings to determine a selected gear position, the second friction element being engaged immediately before the first friction element to be engaged last, the method comprising:
determining whether or not the second piston for actuating the second friction element has completed its stroke up to a second predetermined position where the second piston can start actual engagement; and
preventing, before determination that the second piston has completed its stroke up to the second predetermined position, determination whether or not the first piston for actuating the first friction element has completed its stroke up to a first predetermined position where the first piston can start actual engagement.
Another aspect of the present invention is to provide a system for controlling an automatic transmission with a plurality of friction elements with respective pistons, at least first and second friction elements being engaged at different timings to determine a selected gear position, the second friction element being engaged immediately before the first friction element to be engaged last, the system comprising:
means for determining whether or not the second piston for actuating the second friction element has completed its stroke up to a second predetermined position where the second piston can start actual engagement; and
means for preventing, before determination that the second piston has completed its stroke up to the second predetermined position, determination whether or not the first piston for actuating the first friction element has completed its stroke up to a first predetermined position where the first piston can start actual engagement.